If you backed Rebuild on Kickstarter or preordered direct through rebuildgame.com, you should have gotten an email from humblebundle.com with info on how to pick up a direct copy and your steam key. If you missed the email, you can still get it by visiting here:

And entering your email address. There you can pick up direct download copies, your Steam key, and (if applicable) desktop versions of Rebuild 1 & 2. The PDF artbook will also appear there when the game is finished.